1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image by using an electrophotographic system or an electrostatic recording system, and particularly relates to an image forming apparatus, such as a copying machine, a printer or a facsimile.
2. Related Background Art
An image forming apparatus such as an electrophotographic apparatus (a copying machine, a laser printer or the like) or an electrostatic recording apparatus is known in which: an image forming process that includes a step of conducting electrification processing to an image bearing member is applied to an image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric of rotary drum type or endless belt type, to thereby form an electrostatic latent image corresponding to target image information; the electrostatic latent image is visualized (into a toner image) by a developer; the toner image is transferred to a recording material and further fixed to be output as an image formed material (copy, print); and the image bearing member is repeatedly used for image formation.
In such an image forming apparatus, a corona discharge device has been conventionally and widely used as a means for electrifying the image bearing member (hereinafter referred to as photosensitive member). The corona discharge apparatus is arranged so as to oppose the photosensitive member in a noncontact manner, and the surface of the photosensitive member is exposed to a corona shower emitted from the corona discharge apparatus to which a high voltage is applied, thereby making the photosensitive member surface undergo corona electrification.
In recent years, a contact type electrifying apparatus can attain a lower voltage of a power source in comparison with the corona discharge apparatus, and thus has advantages such as reduced amount of ozone generation. As a result, the contact type electrifying apparatus has been attracting attention and has been put to practical use.
In the contact type electrifying apparatus, a roller type (electrifying roller), a blade type, a fur-brush type, or a magnetic brush type conductive electrifying member (contact electrifying means) is made to contact with a member to be electrified such as the image bearing member, and a predetermined bias voltage is applied to the electrifying member, thereby electrifying the member to be electrified with predetermined polarity and potential.
A method in which a bias applied to the electrifying member is only a direct current voltage (DC application method) and a method in which a bias applied to the electrifying member is a voltage obtained by superimposing a direct current voltage on an alternating voltage (AC application method) are given. Particularly, the latter AC application method has been widely used in recent years since an alternating component makes unevenness of electrification uniform and a direct current component converges a voltage to a predetermined level, thereby easily obtaining uniformity of a surface potential.
Two kinds of contact electrification mechanisms, that is, corona electrification system and direct electrification system (charge injection electrification system), are mixedly present. The characteristic of contact electrification appears depending on which electrification system is dominant.
By the way, in the image forming apparatus using the contact electrifying apparatus, electrification of the photosensitive member must have been previously started before image formation. Specifically, unless electrification has been previously started and performed for at least a period for one peripheral length of the rotating photosensitive member, the electrification potential is unstable, and it is difficult to obtain a satisfactory image with uniform image density. Thus, in the case where a synchronizing means of a recording material is located close to an image forming part, the electrification is required to start at the position of a sensor in an upstream side of the image forming part or with a reference signal. At this time, for example, in the case where a time is required for image processing or in the case where activating of another apparatus, for example, a fixing apparatus in the image forming apparatus is on stand-by, a recording material is always caused to wait at the position of the synchronizing means. If the recording material is not caused to wait at the position for the synchronization, an excess amount of time is required until image formation is completed after the preparation for image formation is completed, whereby an unpleasant feeling is given to a user. That is, if feeding of the recording material is started after preparations for the image forming apparatus are completed, an excess amount of time is needed from the start of image formation until the completion by that amount of time.
Particularly in the image forming apparatus mounted with a memory apparatus for recording image data, for example, in the case where a plurality of sheets of image outputs are obtained based on a sheet of original, or at the time of reading of a double-sided original or at the time of image formation for both surfaces of a sheet, time may be required for image data processing such as accumulation/expansion of memory. When the above-described image formation is performed at this time, a still another excess time will become necessary, thereby increasing a load on the user.
On the other hand, if the recording material is caused to wait at the position of the synchronizing means at this time, an electrification bias is excessively applied to the photosensitive member until the preparation of image data for the first image formation is completed after the accumulation/expansion of memory is completed. As a result, power is consumed unnecessarily.
Here, it has been found through an examination by the present inventors that there exists a proportional correlation between an application time of an electrification bias (particularly AC component) and an abrasion amount of a surface layer of the photosensitive member in the image forming apparatus using the contact electrifying apparatus. Therefore, when the photosensitive member is excessively electrified, abrasion of the surface layer is promoted. Thus, the service life of the photosensitive member becomes short.
That is, there has been a problem in that, when an excess time in the image data processing or in the activating of the image forming apparatus and an excess waiting time of the recording material at the position for synchronization are caused, electric power necessary for electrification is wastefully consumed, which leads to a short life of the photosensitive member.
The present invention has been made in view of the above, and an object of the present invention is therefore to provide an image forming apparatus capable of reducing consumption of power necessary for electrification of an image bearing member as much as possible.
Another object of the present invention is to provide an image forming apparatus capable of extending a life of the image bearing member.
Other objects of the present invention will be apparent by reading the following detailed description with reference to the accompanying drawings.